This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 197 41 019.7, filed on Sep. 18, 1997, of which the entire disclosure is incorporated herein by reference.
The invention relates to a structural material, and particularly a metal based composite material, having a high damping capacity and a high tensile strength, comprising an essentially metallic base material or matrix and a second phase in the matrix. The invention further relates to a method of making such a material and to rigid structural parts made of such a material.
In various fields of industry, the presently typical high accelerations of mechanically moving parts cause undesirable vibrations in those parts over a wide frequency spectrum. The high vibration loading in the vibrating systems leads to long dead or idle times, for example due to long run-up transient processes, and also limits the operating lifetime of the vibrationally loaded parts. Another significant problem is the noise generated by the vibrations.
In order to overcome or avoid these problems of vibrations, it is generally known to use materials having a high damping capacity to damp out the vibrations as much and as quickly as possible. However, present structural materials do not possess a sufficient damping capacity in and of themselves, and present damping materials do not possess a sufficient strength to perform as structural materials themselves.
Metals and metal alloys are predominantly used as structural materials in a broad field of applications, due to their high strength, low weight or density, and good corrosion resistance. However, such metals and metal alloys typically have a rather low damping capacity, so that it becomes necessary to use additional damping materials purely for the purpose of achieving the desired damping in structures comprising metal and metal alloy structural parts. Such damping materials are generally synthetic polymers or plastics, but such materials suffer limitations in their applicability for example in applications at temperatures above the respective melting points of the materials or in situations of limited space. Gray cast iron and pure magnesium are characterized by a higher damping capacity, but on the other hand these materials possess a rather limited strength.
U.S. Pat. No. 4,946,647 (Rohatgi et al.) discloses metallic composite materials of an aluminum matrix with graphite particles dispersed therein as a second phase, as well as a method for making the same. While the known composite materials are said to have an improved damping capacity relative to aluminum or aluminum alloys per se, the disclosed composites have a significantly reduced strength compared to aluminum or aluminum alloys per se. Particularly, the disclosed materials have a tensile strength of at most 190 MPa, which is far below that of the matrix, and an elongation of at most 4%, even when pure aluminum is used as the matrix material. These mechanical properties demonstrate that the disclosed composite materials achieve the desired damping characteristics only at the expense of a drastic reduction in strength, and the materials are thus not suitable for use as structural materials. The reference even admits this deficiency in the strength, and suggests that the graphite content must be controlled or limited to achieve the required strength values. Of course, such limitations on the graphite content will in turn reduce the desired damping characteristics.
U.S. Pat. No. 4,236,925 (Onuki et al.) discloses a method of producing a sintered material having an increased damping capacity and comprising a second phase of graphite, lead or magnesium. The disclosed method includes steps of mixing the second phase material in powder form with the remainder of a powdery iron, copper, or aluminum metal, compression-molding the mixture, sealing or canning the compression-molded mixture in a deformable vessel, subjecting the vessel and the mixture therein to a plastic deformation treatment, and then sintering the plastically deformed vessel and mixture therein. The heating temperature in the sintering step must be above the recrystallization temperature of the matrix metal, so that the matrix metal becomes recrystallized, and the second phase material is aggregated in the form of spindles on the crystal surface or in the crystals of the matrix metal. These steps are complicated and costly, and the method necessarily limits the second phase material to have a spindle shape.
In view of the above, it is an object of the present invention to provide a material that comprises an essentially metallic base material or matrix and a second phase in the matrix, which has both an increased damping capacity already at low vibration amplitudes, as well as a sufficiently high tensile strength and elongation so that it may be used as a structural material particularly for rigid, form-stable structural parts. A further object of the invention is to provide a method for producing a material having a high damping capacity and a high strength, without requiring additional complicated method steps such as high temperature sintering or the like, and without limitation of the form of the second phase material. The invention further aims to avoid or overcome the additional disadvantages of the prior art, and to achieve additional advantages, as apparent from the present description.
The above objects have been achieved in a metallic material according to the invention, having a high damping capacity and a high tensile strength, comprising a metallic base material or matrix and a second phase in the matrix, wherein the second phase is metallic and at least partially comprises a martensitic grain structure. It has been discovered that the inventive combination of a metallic second phase at least partially comprising a martensitic grain structure in a metallic matrix achieves high damping characteristics, even at low vibration amplitudes. A further advantage is that the partially martensitic second phase does not negatively influence the mechanical properties of the matrix, so that the overall material maintains its strength characteristics and therefore can be used as a structural material in the same manner and the same applications as the matrix material could be used by itself.
It is further significant that the inventive material does not place any particular limitations on the selection of the outer shape, form or configuration of the second phase. Namely, the second phase can be in the form of granular or globular particles fibers, strands, whiskers, wires, or the like. By appropriately selecting the form of the second phase, it is possible in a simple manner to adapt the respective characteristics of the overall material to the requirements at hand in any particular application. It should be noted that the second phase is preferably dispersed throughout the matrix in an unmixed and un-alloyed condition relative to the matrix, such that distinct particles of the second phase remain embedded in the matrix, thus providing an overall composite structure for the inventive material. In other words, the overall material according to the invention is especially not a homogenous alloy, but rather a composite of un-mixed materials.
Preferably, the second phase itself is an alloy. An advantageous material damping capacity can be achieved if an alloy of nickel and titanium is used as the second phase, and particularly when each of these alloying components is present and intermixed in the range of 48 to 52 atom %. The most preferred alloy composition of nickel and titanium for the second phase is 49.9 atomic % nickel, and 50.1 atomic % titanium. These components and compositions of the alloy are only preferred values, and are not necessary limitations on the broadest scope of the invention.
The material damping capacity can be even further increased if the second phase includes additives in a positive amount up to 25 atomic %, for stabilizing the martensitic phase and for adapting the material properties to the operating requirements in a particular application. Both the number of additives as well as the compositional content limits thereof are not absolute limitations on the broadest scope of the invention, but represent preferred values. The additives may advantageously be selected from among zirconium, hafnium, copper, niobium, manganese, palladium, platinum and iron. Stabilization of the martensitic phase can be further reinforced by a pretreatment of the second phase material, for example by plastically deforming the second phase or carrying out a homogenizing treatment of the alloy components of the second phase.
As mentioned above, the second phase can be in the form of granular or globular particles, grains, short fibers, long fibers, whiskers, spheroids, or the like. Throughout this disclosure, the general term xe2x80x9cparticlesxe2x80x9d will be used to cover all different configurations or forms of the particles, grains, fibers, etc., unless the particle shape is particularly specified. The second phase particles can be uniformly dispersed throughout the matrix, or may be present or concentrated in one or more layers extending through the matrix material. Particles having an anisotropic shape, such as fibers or whiskers, may be randomly oriented or oriented in a uniform direction. By properly selecting the particle form, dispersion pattern through the matrix, and particle orientation, the overall properties of the resulting composite material can be maximally adapted to the externally specified requirements.
This can further be achieved in that the proportional content of the second phase in the total material is varied or selected, preferably in the range from 5 to 60 vol. %, depending on the desired overall properties of the material. A higher proportion of the second phase will generally achieve a higher damping capacity. Therefore, in applications where damping is particularly important, the invention provides for greater than 30 vol. % and particularly greater than 35 vol. % of the second phase. Once again, these ranges are preferred proportional ranges providing improved characteristics, but are not strict limitations on the broadest scope of the invention.
In order to improve the bonding of the second phase with the matrix material, and thereby to better ensure the necessary load transfer from the matrix material to the second phase for achieving the desired damping, it is advantageous if an outer boundary layer of each particle of the second phase material forms a compound or a mixed phase with the matrix material. Such a structure can be achieved by carrying out the method according to the invention as discussed below.
By providing a metallic second phase that has an at least partially martensitic grain structure within the matrix material, it is possible to achieve the required damping characteristics of the overall composite material by the function or action of the second phase by itself, while the tensile strength and elongation of the overall material are predominantly provided or determined by the matrix. Namely, the overall composite material preferably has a minimum tensile strength and minimum elongation of 10%, or preferably only 5%, below the corresponding values of the matrix material by itself. More preferably, the tensile strength and elongation of the overall composite are equal to or even better than those of the matrix material by itself. For this reason, the matrix material can be selected accordingly to achieve all requirements as a structural material in different applications, while the second phase can be selected to achieve the desired damping characteristics. The overall composite material has a damping capacity of greater than 1xc3x9710xe2x88x923 and an elongation greater than 1%. The damping capacity of the overall composite material is preferably at least 10 times that of the matrix material by itself.
In view of the mechanical properties, the base material is advantageously a light metal or a light metal alloy, such as aluminum, aluminum alloys, titanium, titanium alloys, or magnesium alloys for example, and excluding iron-based alloys. In this context, it is particularly preferred to use the aluminum alloy designated as EN AW-6061 according to the German Industrial Standard DIN EN 573, which has the following composition: 0.40 to 0.8 wt. % of silicon, up to 0.7 wt. % of iron, 0.15 to 0.40 wt. % of copper, up to 0.15 wt. % of manganese, 0.8 to 1.2 wt. % of magnesium, 0.04 to 0.35 wt. % of chromium, up to 0.25 wt. % of zinc, up to 0.15 wt. % of titanium, up to 0.15 wt. % total of other additives, and the remainder of aluminum. As an example, a sample having 90 vol. % of a matrix of aluminum EN AW-6061 and 10 vol. % of a second phase of Ni49.9Ti50.1 achieves a tensile strength of 223 MPa, a damping capacity of 5xc3x9710xe2x88x923 and an elongation of more than 10%. However, applications that require an even higher strength may exist or arise, in which case it is possible to use a matrix material that has different or various grain structures or that is a composite material in itself including a matrix and a third phase dispersed in the matrix for reinforcement. A thermomechanical treatment properly adapted for the particular matrix material can similarly lead to an advantageous increase of the strength of the matrix material.
The above objects have further been achieved by a method of making a material having high damping capacity and high strength, according to the invention, comprising steps of mixing a powdery matrix material and a powdery second phase material, and then heat treating the mixture in a temperature range from 400 to 700xc2x0 C. and consolidating the same at a pressure of 100 to 300 MPa. In this context, the holding time can be between one hour and six hours. However, carrying out the consolidation at 540xc2x0 C. and 200 MPa for a two hour holding time has been shown to achieve the best results.
With regard to the selection of the characteristics of the starting or initial states of the matrix material and of the second phase, the best results have been obtained when the matrix material is in the form of a particulate or powder material that has been rapidly solidified by atomizing it into an inert gas environment. Insofar as the selected outer configuration of the second phase particles allows it, the second phase material is also provided in the form of particles that have been processed in this manner. An advantageous additional method step involves de-gassing the mixture of matrix material and second phase material before carrying out the consolidation. The consolidation itself can be carried out by means of hot isostatic pressing, sintering, extrusion pressing, or forging.
As a further variation of the method according to the invention, the matrix material may first be melted, and the second phase material may then be mixed into the molten matrix material. In this case, it is advantageous to protect the second phase particles in order to prevent an excessive reaction between the two materials when the second phase particles are mixed into the molten matrix material. This can be achieved by providing a coating on the outside surface of the respective second phase particles before introducing the particles into the molten matrix material.
In both variants of the inventive manufacturing method, i.e. either using a powdery matrix material or using a molten matrix material as a starting point, the second phase particles can be provided in various external configurations. Advantageously, the second phase is provided in the form of globular or granular particles, wires, short fibers, or one or more layers. In a further preferred step, the second phase is pre-treated in order to stabilize the martensitic phase before carrying out the consolidation. This martensite stabilizing treatment can involve deforming the second phase material, or homogenizing the components of the second phase therein.
The methods for manufacturing the material according to the invention do not necessarily include any critical method steps in which the second phase material must be brought into a particular external form or configuration. Moreover, the present methods do not require an additional step of heating the material to a temperature above the recrystallization point of the matrix material in order to achieve a high damping capacity.